System for cooling hybrid vehicle electronics, method for cooling hybrid vehicle electronics

ABSTRACT

The invention provides a single radiator cooling system for use in hybrid electric vehicles, the system comprising a surface in thermal communication with electronics, and subcooled boiling fluid contacting the surface. The invention also provides a single radiator method for simultaneously cooling electronics and an internal combustion engine in a hybrid electric vehicle, the method comprising separating a coolant fluid into a first portion and a second portion; directing the first portion to the electronics and the second portion to the internal combustion engine for a time sufficient to maintain the temperature of the electronics at or below 175° C.; combining the first and second portion to reestablish the coolant fluid; and treating the reestablished coolant fluid to the single radiator for a time sufficient to decrease the temperature of the reestablished coolant fluid to the temperature it had before separation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority as a divisional of U.S.patent application Ser. No. 14/596,415, filed on Jan. 14, 2015,currently pending.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. DE-ACO2-06CH11357 between the U.S. Department of Energy and UChicagoArgonne, LLC, representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a system for cooling electronics, and morespecifically this invention relates to a compact system and method forcooling hybrid vehicle electronics using only one radiator.

2. Background of the Invention

Hybrid vehicle electronics have become more sophisticated. As a result,the use of wide-bandgap semiconductors will increase. Wide-bandgapsemiconductors permit devices to operate at much higher voltages,frequencies and temperatures than conventional semiconductor materials.This allows for more powerful electrical mechanisms to be built whichare cheaper and more energy efficient.

“Wide-bandgap” refers to higher voltage electronic band gapssignificantly larger than one electron volt (eV). The exact threshold of“wideness” often depends on the context, but for common usage, “wide”bandgap typically refers to material with a band gap of at least 3 eV,significantly greater than that of the commonly used semiconductors,silicon (1.1 eV) or gallium arsenide (1.4 eV).

Wide-bandgap materials are often utilized in applications in whichhigh-temperature operation is important. The higher energy gap gives thedevices the ability to operate at higher temperatures. However, ajunction temperature of between 150° C. and 175° C. should be maintainedunder the semiconductors to prevent electronics malfunction. This cannotbe accomplished with 105° C. coolant used in standard radiators.

Automotive examples of wide-bandgap devices include traction drivecomponents, battery chargers (for plug in hybrid electric vehicles,PHEVs), boost converters (for stepping up battery voltages higher thanthe battery capacities), inverters (for converting DC to AC for phasedpower to traction motors and generators), and bi-directional DC-DCconverters (to shuttle power among buses to operate lighting, brakeassist, power steering, etc.).

State of the art power electronic semiconductors in hybrid vehiclesattempt to address high temperatures using multiple heat exchangers orradiators. Typical heat sink configurations consist of multiple layersof materials, starting with the semiconductors, followed by a copperthermal spreader, one or more layers of a thermal interface material(TIM), and flow channels for the liquid coolant.

FIG. 1 is a perspective view of a prior art cooling configuration, thatconfiguration designated as numeral 9. One or a plurality ofsemiconductors 12 are supported on a thermal spreading substrate 14. Thethermal spreading substrate 14 is supported by one or a plurality of TIM16. This TIM 16 is in physical contact with flow channels 18 adapted toreceive coolant fluids.

Each of the layers below the semiconductors of FIG. 1 has a resistanceto heat transfer. The largest of these resistances are the TIMs 16 andthe coolant fluid. Coolant fluids exhibit high resistance to heattransfer due to their laminar flow characteristics. Such poor laminarflow heat transfer rates require that a second radiator (using 75° C.coolant) be used in hybrid electric vehicles to cool the powerelectronics. As such, state of the art hybrid vehicle cooling systemsutilize two separate radiators, one for the internal combustion engine,and a second one for the electronics. This second radiator andassociated plumping adds cost and weight to the vehicle while reducingavailable space for other components.

A need exists in the art for an electronics cooling system and methodthat does not employ multiple radiators. The system and method shouldeliminate or substantially reduce the thermal resistance now plaguingstate of the art coolant-side fluid dynamics, such that the system andmethod eliminates the potential of a TIM reaching a CHF condition. Thesystem and method should maintain the electronics side at no more thanapproximately 175° C., given power production rates of state of the artchips of about 100 W/cm², while minimizing pumping power requirements.

SUMMARY OF INVENTION

An object of the invention is to provide an electronics cooling systemthat overcomes many of the disadvantages of the prior art.

Another object of the invention is to provide a system and method forcooling hybrid vehicle electronics. A feature of the invention is thatonly one radiator is required to cool both the internal combustionengine and the electronics of the vehicle. Another feature of theinvention is that the invention can be configured to cool a singlesurface, or a plurality of surfaces of the electronics components. Anadvantage of the invention is that it confers lighter weight andincreased compactness.

Still another object of the present invention is to provide a compactsystem for cooling hybrid vehicle electronics. A feature of theinvention is that it uses most of the same components of typical coolingsystems. An advantage of the invention is that it provides increasedcooling rates at low pumping power, therefore leading to reduced costsand weight.

Yet another object of the present invention is to provide a method forcooling wide-bandgap semiconductor electronics. A feature of theinvention is that it eliminates the need for cooling fins, otherwise inthermal communication with the electronics and foundation plates. Anadvantage of the present invention is its ability to accommodate powerdensities of at least about 100 W/cm² without cooling fins and as highas about 250 W/cm² with fins (multiple-sided cooling), therefore findingapplicability to new hybrid electric vehicles with, and without,wide-band semiconductors.

Another object of the present invention is to provide approximately a 25percent more efficient method and system for cooling high powerelectronics. For example, the present invention can optimize a current,one-sided, cooling system from a typical 100 W/cm² power density to 125.A feature of the invention is the use of traditional vehicle coolantsthat are subcooled, such that the coolants remain as substantially asingle phase throughout the cooling cycle. An advantage of the inventedboiling coolant method and system is its superior heat removal capacitycompared to traditional laminar fluid flow convective heat transfersystems, such that the invented system facilitates more efficientcooling of high power density electronics.

Still another object of the present invention is to provide a singleradiator system that combines internal combustion engine cooling andpower electronics cooling operations, whereby the system can maintain250 W/cm² density electronics at or below 175° C. A feature of theinvention is that a plurality of typical electronic heat sink surfaces(e.g. two sides of a semiconductor chip) are contacted with pressurizedtypical engine coolant. An advantage of the invention is that thepressurized fluid provides an adequate subcooling temperature range forkeeping junction temperatures of advanced power electronics withinoperating limits. The system is passive in that it does not requirecooling jets, nozzles, gas separators or other moving means for coolingheated surfaces via atomization (e.g. spray cooling), forced convection(e.g. pressurized), or other mechanical means of mass flow of a fluidsuch as liquid coolant or gas such as air or refrigerant. Rather, theengine coolant's resistance to heat transfer is substantially decreasedby the subcooled boiling. As such, an embodiment of the invention isnozzle-less.

Briefly, the invention provides a single radiator cooling system for usein hybrid electric vehicles, the system comprising a surface in thermalcommunication with electronics, and subcooled boiling fluid contactingthe surface.

The invention also provides a single radiator method for simultaneouslycooling electronics and an internal combustion engine in a hybridelectric vehicle, the method comprising separating a coolant fluid intoa first portion and a second portion; directing the first portion to theelectronics and the second portion to the internal combustion engine fora time sufficient to maintain the temperature of the electronics at orbelow 175° C.; combining the first and second portion to reestablish thecoolant fluid; and treating the reestablished coolant fluid to thesingle radiator for a time sufficient to decrease the temperature of thereestablished coolant fluid to maintain steady state cooling in bothportions. In an embodiment of the method, the coolant is maintainedthroughout the process at a temperature below its boiling point and thereestablished coolant has approximately the same temperature enteringthe radiator as in a conventional single-phase cooling loop.

BRIEF DESCRIPTION OF DRAWING

The invention together with the above and other objects and advantageswill be best understood from the following detailed description of thepreferred embodiment of the invention shown in the accompanyingdrawings, wherein:

FIG. 1 is a perspective view of a prior art electronics coolingconfiguration;

FIG. 2 is a schematic view of a subcooled boiling cooling system, inaccordance with features of the present invention;

FIG. 3 is a graph depicting temperatures of various regions of coolantfluid, in accordance with features of the present invention;

FIG. 4 is a graph of temperatures in various regions of exemplary powerelectronics treated with subcooled boiling systems, in accordance withfeatures of the present invention;

FIG. 5 is a graph comparing coolant flow velocity to electronicsjunction temperature, in accordance with features of the presentinvention;

FIG. 6 is a graph comparing coolant inlet temperature to electronicsjunction temperature, in accordance with features of the presentinvention;

FIG. 7 is a graph depicting junction temperature as a function of heatflux, in accordance with features of the present invention; and

FIG. 8 is a graph comparing measured temperatures with mathematicallypredicted temperatures in a subcooled boiling, in accordance withfeatures of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralsaid elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising” or “having”an element or a plurality of elements having a particular property mayinclude additional such elements not having that property.

The invented subcooled boiling electronics cooling-system and methodutilizes subcooled (e.g. compressed) boiling or low vapor qualitysaturation boiling of cooling fluids to enhance cooling of powerelectronics. A salient feature of the invention is a controlled use ofboiling of coolant fluid to increase the accuracy in thermal managementof these electronics. As such, the invention utilizes non-laminarcoolant flow heat transfer paradigms to provide superior heat sinkcharacteristics.

A myriad of fluids are suitable for use in the invented system,including, but not limited to glycol based liquids (e.g. ethyleneglycol, polyethylene glycol, propylene glycol, water, polyalphaolefin(PAO), and combinations thereof. The venue of the semiconductors to becooled will determine the fluid utilized. For example, in automotiveapplications, the semiconductors used in hybrid electronics should bemaintained at or below 175° C. In such automotive applications, waterand its mixtures containing ethylene glycol (0 to 60 percent massfraction, limited by this maximum allowable temperature) is a suitablecoolant.

An embodiment of the invention is applicable using components alreadyfound in hybrid electric vehicles. The invention requires only oneradiator and coolant pumping system while maintaining no more than 175°C. semiconductor temperatures. There are several ways to do this:

-   -   1. If costly fins are not utilized as the coolant passages in        the power electronics, current heat loads of 100 W/cm² are        removed with the use of double-sided cooling.    -   2. If typical heat sink topographies are utilized (e.g., fins        are utilized), 125 W/cm² can be removed with single-sided        cooling. This allows for approximately a 25 percent increase in        semiconductor size (power output).    -   3. If fins and double-sided cooling are utilized, approximately        250 W/cm² (i.e., the power load of wide-band semiconductors) can        be removed. Alternatively, fins used with double-sided cooling        configurations can cool electronics substantially below 175° C.        when less than 250 W/cm² densities exist. FIG. 7, discussed        further infra, provides cooling data for single sided cooling        with fins. As such, when double sided, fin cooling is utilized,        the heat flux values are doubled for a given junction        temperature.

Given the above configurations, the invention is particularlyadvantageous in attaining desired cooling ranges utilizing alreadyexisting technology in vehicles. No other components than those found inexisting hybrid vehicles are required. For example, the microstructureof the TIMs utilized in the invention are unadulterated (uncoated,substantially conformal and nonporous) in that they define typicalmorphologies. As such, the TIMs lack any special surface finishes,coatings (such as microporous coatings) or generally enhanced porositiesor surface area enhancing topographies other than those that aregenerated during typical production of TIM substrates.

An embodiment of the invention is that it can utilize subcooled boilingalone, i.e. without bulk boiling and without net vapor generation. Withsubcooled boiling, any vapor generated at the hot surfaces of thecoolant channels collapses in the cooler fluid in the center of thechannels and leaves the power electronics bathed in liquid phase coolantexclusively. Therefore, the only place boiling occurs is at theinterface of the heatsink of power electronics. The remainder of thecoolant system operates with liquid coolant.

Alternatively, the invention is operational with some net vapor leavingthe heatsink surfaces, where it initially forms. The leaving vapor thencombines with cooler liquid recirculating back from the engine such thatthe resulting reestablished coolant is solely liquid phase once again.Therefore, the invention can operate with bulk boiling in the powerelectronics if substantially the entire volume of vapor generated isempirically determined to completely change phase back to liquid whencombined with the engine coolant. Subcooled boiling case minimizes theportion of the cooling system in which boiling occurs.

Another cooling scenario enabled by the instant method is theutilization primarily of bulk boiling. When bulk boiling occurs at theexit of the power electronics package, subcooled boiling could precedeit upstream in the package or it could be mostly or all bulk boiling. Inany case when bulk boiling occurs at the exit, the coolant is at itsboiling temperature with some vapor mixted with liquid.

When the temperature difference between the thermal transfer surface andthe coolant saturation T_(sat) is greater than approximately 4° C. (7.2°F.) to 10° C. (18° F.), isolated bubbles form at nucleation sites andseparate from the surface. This separation induces considerable fluidmixing near the surface, thereby providing an automatic means forsubstantially increasing the convective heat transfer coefficient andthe heat flux. The coolant is boiling at the surface (surfacetemp>T_(sat)) while the coolant bulk temperature is below its saturationpoint. So, the system is in subcooled boiling with the heat transferadvantages. Therefore, in an embodiment of the invention, single phaseconvection is not the means for transferring heat from electronicssurfaces. In another embodiment of the invention, single-phaseconvection plays a role in heat transfer.

In an embodiment of the invention, traditional vehicle engine coolantsare utilized, such as 50/50 ethylene glycol/water mixtures, at pressuresbetween about 0 psig and about 45 psig (gauge pressure, i.e., aboveatmospheric pressure). Other coolants are also suitable, such as water,propylene glycol, and combinations thereof. The coolant system defines aclosed system, whereby coolant is only added to the system if there is aleak or loss. The system is adaptable to a myriad of different coolingparadigms, such that spent or unwanted coolant maybe drained from thesystem, and replaced with fresh coolant or different coolant. Theradiator often serves as the coolant reservoir. As such, draining andfilling are often done through a valve at the bottom of the radiator andthe cap on top of the radiator, respectively.

An embodiment of the invention is the closed system shown schematicallyas numeral 20 in FIG. 2. Coolant is pumped, pressurized, and otherwisemanipulated by a pump 22 in the closed system. Coolant pressures inautomobiles are usually limited to about 15 psig, with truck pressureshigher than automobiles (e.g., about 30 to about 45 psig). Generally,pressures are substantially constant throughout the system but may behighest by a few psig at the outlet of the pump, and lowest at the inletof the pump. The maximum pressure is limited by pressure relief valvesin the coolant system.

Upon treatment with the pump 22, the fluid contacts a flow divider 24situated downstream from the pump 22. The flow divider 24 is situatedbetween the power electronics 26 and the regular internal combustionengine 28 of the vehicle.

The flow divider 24 separates the coolant fluid into two portions whichmay or may not be equal in volume. The relative sizes of the portionswill depend on the size of the combustion engine and the electronics bayrequiring cooling. In one embodiment, the divider 24 separates thecoolant fluid volume into approximately two equal portions. A firstportion follows a typical combustion engine cooling route 30. A secondportion 32 is directed to the cold plate of the power electronics, thecold plate 16 depicted in FIG. 1 defining the fluid channels 18. Itshould be noted that while a typical cold plate is integrally moldedwith (so as to be in thermal contact with) fins defining the fluidchannels 18, an embodiment of the invention eliminates the need forfins, given the extremely efficient heat transfer characteristics of theinvented subcooled boiling paradigm. As such, electronic bays withsmooth (e.g. fin-less) heat transfer surfaces in contact with coolantfluid are adequately cooled with the invented system.

Inasmuch as the cold plate is in thermal communication with theelectronics, the coolant in contact with the cold plate 16 absorbs heatfrom the power electronics, at the maximum heat flux, predominatelythrough subcooled boiling or low vapor quality saturation boiling. Thecoolant channels may have some single-phase heat transfer at theextrance even at the maximum heat rates. When the vehicle is operatingat less than the maximum heat rate, there will be more, or all,single-phase heat transfer.

Downstream from the combustion engine 28 and the power electronics 26,the two fluid portions recombine via a mixer 36 to form a uniformtemperature fluid 38. The uniform temperature fluid 38 is then directed,still under pressure, to a vehicle radiator 40 where heat exchangersthere cool the fluid. Upon exiting the radiator 40, the coolant returnsto the pump 22 to complete the loop.

During subcooled boiling, the coolant bulk temperature is lower than thefluid saturation temperature and the wall temperature is higher than thefluid saturation temperature. This is depicted in FIG. 3, wherein thetemperature of the bulk coolant, T_(f), is below the fluid saturationtemperature (boiling temperature), T_(sat), in both the single-phase andsubcooled boiling regions. Also shown in FIG. 3 is the wall temperature,T_(w), which exceeds the fluid saturation temperature at the end of thesingle-phase region. Then, subcooled boiling commences when the walltemperature is sufficiently above the saturation temperature as shown inFIG. 3. The sketch of FIG. 3 is a representation of conditions thatoccur in a typical channel heated all around. In the cooling channels ofpower electronics packages, heat is transferred to the coolant from oneside, i.e. from the top in a conventional package with single-sidedcooling, or from the bottom of the channels in the top cooling plate ina double-sided cooling configuration. Although the vapor/liquid patternsdiffer among these three heating paths, the explanation of thetemperatures given above for FIG. 3 is applicable to all cases.

Vapor bubbles generate on the hot wall surfaces but collapse in therelatively cold fluid. Heat is transferred from the surface to thecoolant in the form of vapor bubbles. As the bubbles move into thecenter of the channel, they collapse in the cool fluid, and theytransfer the heat from the wall to the fluid. So, the fluid increases intemperature as it flows through the channels (from left to right in FIG.3) just as would a single-phase liquid.

In a preferred system operation, no net vapor is generated from thecirculating coolant, and the fluid remains a single phase at the exitpoint of the cooling channel. Under these conditions, subcooled boilingexists in the power electronics. (There may be some single-phase liquidheat transfer at the entrance to the power electronics even whensubcooled boiling exists over the remainder (majority) of the surface.)The coolant enters the power electronics from the left in FIG. 3 assubcooled single-phase liquid. It may exit as subcooled liquid (with orwithout subcooled boiling having occurred), or as saturated coolant withor without vapor depending if bulk boiling occurred. The system may bedesigned not to reach bulk boiling, but single-phase heat transfer andsubcooled boiling will occur in different amounts depending on the heatoutput from the power electronics.

For low vapor quality saturation boiling, the coolant bulk temperatureis at the fluid saturation temperature, and the wall temperature ishigher than the fluid saturation temperature. As such, vapor bubblesgenerate on the hot wall surfaces and enter the mixer 36 with thecoolant flow. However, since the coolant flow rate for cooling the powerelectronics is generally lower than that for cooling the engine, and theengine coolant temperature is below the fluid saturation temperature,the vapor generated from saturation boiling condenses in the mixer,resulting in a combined flow which is a single-phase fluid.

During either subcooled boiling or low vapor quality saturation boiling,the coolant fluid remains substantially a liquid throughout the coolingand heating cycle. In subcooled boiling, the only vapor occurs in thepower electronics channels (e.g. in close spatial relation to the heatsink surface of the power electronics); the remainder of all fluidpassages, conduits, mixers, componentry and other structures arecompletely filled with liquid.

In saturated boiling (also called bulk boiling), in addition to vaporcontacting the heat sink surfaces, there is also vapor in the conduitbetween the power electronics and the mixer (36 in FIG. 2);simultaneously though, the coolant exists substantially only as a liquideverywhere else in the system. The percent of coolant by mass that isvapor is called the “quality.” For subcooled boiling, the coolant exitsthe power electronics subcooled so the quality is zero. For saturatedboiling, the coolant exits with a low quality. The exact value varieswith the flows, but it is low enough so that when it mixes with theengine coolant, in the mixer, the mixture is all liquid. There is nogeneral limit on the quality.

EXAMPLE

As noted supra, an embodiment of the invented system and method utilizestypical anti-freeze, anti-boil fluids found in internal combustionengine paradigms. The main engine cooling system works at about 2 atmabsolute or 1 atm gauge (approximately 15 psig) of pressure. Acorresponding saturation temperature for the 50/50 ethylene glycol/watermixture is about 129° C. Simulations show that there is enough of asubcooled range for keeping the juncture temperature of powerelectronics within the preferred aforementioned 150-175° C. window.

Simulations by the inventors revealed the conditions under which thecoolant exits the power electronic channels and is still be below thesaturation point. Exemplary conditions include the following:

-   -   Because the subcooled boiling system is integrated into the main        engine cooling system, the conventional engine coolant, a 50/50        EG/W mixture, is used for power electronics cooling.    -   The pressure in the cooling channel for power electronics in        hybrid electric vehicles (HEV) is 2 atm where the saturation        point of a 50/50 EG/W mixture is 129° C.    -   In order to eliminate the low-temperature radiator and the        associated pumping system, the coolant inlet temperature is        assumed to be 105° C.    -   The coolant flow velocity is 0.16 m/s in order to keep the        coolant outlet temperature below the saturation point and to        generate desired subcooled boiling. Liquid flow in the cooling        channels is laminar at this flow velocity.    -   The coolant outlet temperature is below the saturation point.        Therefore, there is no net vapor in the rest of the system        (outside the power electronics cooling channels).    -   To have desired subcooled boiling, the cooling channel wall        temperature is 10-30° C. above the saturation point, i.e. a wall        superheat of 10-30° C.

Under such conditions, subcooled boiling would exist throughout thepower electronics.

Software Option Detail

The single radiator relied upon in the invented system is a heatrejecting device and operates under similar conditions to a typicalradiator which cools only an internal combustion engine. In anembodiment of the invention, software (e.g., COMSOL MultiphysicsModeling Software, by AltaSim Technologies, Columbus, Ohio, USA) isutilized to determine flow of coolant that is needed to maintain thepower electronics at or below 175° C. (Alternatively, coolant flow canbe determined empirically.)

The software is applied to numerical simulations using computationalfluid dynamics (CFD) and heat transfer modules. Analysis of heattransfer was taken along line 4-4 of FIG. 1, i.e., through a verticalplane including the center of both semiconductors 12. Inasmuch as thehottest spot under the semiconductors is in its center, the selectedsoftware paradigm represents the worst situation of heat dissipation inthe power electronics package. This ensures that the conditions runningsimulations maintained the electronics at or below 175° C. even in mostconservative (i.e. the hottest or worst case) situations.

Typical results from the software simulations are shown in FIG. 4. Thetemperatures are shown as different shades of gray from white (175° C.)to black (138° C.). FIG. 4 is a view of FIG. 1 taken along line 4-4 fordouble-sided subcooled boiling heat transfer without fins in channelstypical in size for automotive power electronics cooling passages.Parameters varied in the simulations include the TIM thermal resistance,the power density of the power electronics, and the type of cooling,subcooled boiling (e.g. non-laminar) or laminar flow. As noted supra,conventional laminar flow single-sided cooling cannot adequately coolthe power electronics at 100 W/cm² with a single high temperature (105°C.) radiator. In this conventional situation, a separate radiator (75°C.) is required to maintain the semiconductor junction temperature below175° C.

FIG. 4 depicts results for a current power density of 100 W/cm² withoutfins and with double-sided cooling. The right side of the graph showsthe temperature of substantially the entire electronics construct beingmaintained at between about 135° C. and about 175° C., without the needfor integrally molded cooling fins. This preferred temperature range isthe result of subcooled boiling in the cooling passages. When higherpower density power electronics are made available, doubled sidedcooling with subcooled boiling will increase (typically from at leastabout 25 percent to 100 percent) the heat removal rate. The inventionincreases heat removal rate up to 150 percent. For example, doubledsided cooling with subcooled boiling and fins will increase the heatremoval rate to 250 W/cm² while maintaining the electronics at or below175° C. It is noteworthy that the system depicted in FIG. 4 utilized asingle passage cooling system, which is to say a smooth, uninterruptedcooling plate devoid of fins or other types of multiple cooling channelconfigurations. This reduces both manufacturing costs and pumping powercosts.

In instances where heat transfer coefficients are utilized, suchcoefficients are determined by the Shah 1977 correlation, as waspublically disclosed in ASHRAE Transcripts, 83(1) 1977, the entirety ofwhich is incorporated by reference. (ASHRAE was formerly known as theAmerican Society of Heating, Refrigerating and Air ConditioningEngineers.) The derived coefficient is then used by the software todetermine flow rate.

FIG. 5 is a plot of the junction temperature versus the coolant flowvelocity. It is for a double-sided cooling system with a 7.5-W/m K TIMthermal conductivity for a 100-W/cm² heat flux on the insulated-gatebipolar transistor (IGBT) and diode surfaces. The coolant flow inlettemperature is 105° C. It is seen that the finned channels combined withsubcooled boiling can reduce the junction temperature below 140° C. forall coolant velocities. Without fins, the junction temperature can becontrolled below 175° C. when the coolant flow velocity is 0.16 m/s orhigher. FIG. 5 shows that the coolant flow velocity does not havesignificant effect on the junction temperature for the subcooled boilingsystem.

Efficient cooling using subcooled boiling occurs at low coolant flowvelocities, which reduces pressure drops and pumping power requirements.Using fins in the cooling channels, the coolant flow velocity range forsubcooled boiling is between 0.06 m/s to 0.4 m/s (the range of FIG. 5).When the velocity is lower than the lower limit of this range, thecoolant outlet temperature would be likely above the saturation point.When the velocity is higher than the higher limit of this range, thecooling channel wall temperature cannot reach 10° C. above thesaturation point and therefore subcooled boiling is unlikely to occur.The subcooled boiling pressure drop along the cooling channel isrelatively small (below approximately 1500 Pa, and preferablyapproximately 1440-1450 Pa), which would result in low pumping powerrequirements. Often, approximately zero pressure drop is present.

FIG. 6 illustrates coolant inlet temperature effects on subcooledboiling. A double-sided cooling system with a 7.5-W/m K TIM thermalconductivity for a 100-W/cm² heat flux on the IGBT and diode surfaceswas also considered in this test. FIG. 6 shows that the junctiontemperature can be controlled below 175° C. without fins in the coolingchannel and below 150° C. with fins when using subcooled boiling.

The trends in FIG. 6 are different with and without fins. Without fins,the subcooled boiling dominates the cooling process. High coolant inlettemperatures cause strong subcooled boiling due to high subcooledboiling heat transfer coefficients. Therefore, a higher coolanttemperature results in a lower junction temperature. (This seeminglycounterintuitive phenomenon is due to the resistance to heat transferduring boiling being so low, such that differences in fluid temperatureand fluid velocity are secondary effects on the heat transfer rate.)With fins in the cooling channel, convective heat transfer is alsoimportant. Consequently, a higher coolant temperature results in ahigher junction temperature.

Preferably, in order to maintain subcooled boiling in the coolingchannels, the coolant flow inlet temperature should not be below 100° C.with fins while the coolant inlet temperature should not be below 90° C.without fins because lower coolant inlet temperatures cause the channelwall temperature to be below the subcooled boiling range. Furthermore,according to the simulation results displayed in FIG. 6, the coolantinlet temperature does not have significant effects on the junctiontemperature, especially for the non-finned cooling channel.

FIG. 7 is a graph depicting junction temperature as a function of heatflux, in accordance with features of the present invention. The coolantutilized for this graph was a 50/50 ethylene glycol/water mixture. Thecoolant inlet temperature was about 105 C (which is the temperature oftypical radiator fluid in an internal combustion engine system. Flowrate was 0.16 m/s. The vertical dashed line is the separation pointbetween single phase flow and subcooled boiling flow. These data pertainto a finned system.

FIG. 8 is a graphical comparison between experimentally measuredtemperatures and temperatures calculated using the aforementioned Shahcorrelation, as discussed supra. The good temperature comparisonsupports the use of the Shah correlation in the computer simulationsperformed in the development of this invention.

In FIG. 8, values a-e are heat input values ranging from betweenapproximately 70 W and 140 W. In one embodiment of the invention, with aT_(f) of approximately 82° C., a=74 W, b=67 W, c=80 W, d=130-131 W ande=139 W.

In summary, the invented system enhances the cooling capacity for powerelectronics using two-phase subcooled boiling in the cooling channelswhile the coolant outlet temperature is still below the saturationpoint. Thus, there is no vapor in the rest of the system.

It is to be understood that the above description is intended to beillustrative, and not restrictive. The above-described embodiments(and/or aspects thereof) may be used in combination with each other. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from itsscope.

While the dimensions and types of materials described herein areintended to define the parameters of the invention, they are by no meanslimiting, but are instead exemplary embodiments. Many other embodimentswill be apparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled. In the appended claims,the terms “including” and “in which” are used as the plain-Englishequivalents of the terms “comprising” and “wherein.” Moreover, in thefollowing claims, the terms “first,” “second,” and “third,” are usedmerely as labels, and are not intended to impose numerical requirementson their objects. Further, the limitations of the following claims arenot written in means-plus-function format and are not intended to beinterpreted based on 35 U.S.C. §112, sixth paragraph, unless and untilsuch claim limitations expressly use the phrase “means for” followed bya statement of function void of further structure.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” “more than”and the like include the number recited and refer to ranges which can besubsequently broken down into subranges as discussed above. In the samemanner, all ratios disclosed herein also include all subratios fallingwithin the broader ratio.

One skilled in the art will also readily recognize that where membersare grouped together in a common manner, such as in a Markush group, thepresent invention encompasses not only the entire group listed as awhole, but each member of the group individually and all possiblesubgroups of the main group. Accordingly, for all purposes, the presentinvention encompasses not only the main group, but also the main groupabsent one or more of the group members. The present invention alsoenvisages the explicit exclusion of one or more of any of the groupmembers in the claimed invention.

The embodiment of the invention in which an exclusive property orprivilege is claimed is defined as follows:
 1. A single radiator coolingsystem for use in hybrid electric vehicles, the system comprising: a) asurface in thermal communication with electronics; b) subcooled boilingfluid contacting the surface.
 2. The system as recited in claim 1wherein the fluid is pressurized.
 3. The system as recited in claim 1wherein the surface is smooth and continuous.
 4. The system as recitedin claim 1 wherein the fluid is a liquid selected from the groupconsisting of ethylene glycol, propylene glycol, water, and combinationsthereof.
 5. The system as recited in claim 1 wherein the fluid ispressurized from between approximately 0 psig and approximately 45 psig.6. The system as recited in claim 1 wherein the subcooled boiling fluidcontacts a plurality of surfaces in thermal communication with theelectronics.
 7. The system as recited in claim 1 wherein the surfacedefines a single fluid passage.
 8. The system as recited in claim 1wherein the single radiator cools both the electronics and an internalcombustion engine.
 9. The system as recited in claim 1 wherein theelectronics are maintained at a temperature of less than about 175° C.10. The system as recited in claim 8 further comprising an algorithm foroptimizing fluid flow to the surface.